1. Field
The present invention relates to micro-electromechanical systems (MEMS) and, in particular, to a micromachined electromechanical radio frequency (RF) switch that can preferably function over a range of signal frequencies from 0 Hz to approximately 100 GHz.
2. Description of Related Art
MEMS (micro-electromechanical system) switches have a wide variety of uses in both military and commercial applications. For example, electrostatically actuated micro-electromechanical switches can conduct RF current in applications involving the use of antenna phase shifters, in the tuning of reconfigurable antenna elements, and in the fabrication of tunable filters.
A representative example of a prior art MEMS switch is disclosed in Yao, U.S. Pat. No. 5,578,976, issued Nov. 26, 1996. Typically, this type of MEMS switch is fabricated on a semi-insulating substrate with a suspended micro-beam element as a cantilevered actuator arm. The cantilever arm is coupled to the substrate and extends parallel to the substrate, projecting over a ground line and a gapped signal line formed by metal microstrips on the substrate. A metal contact, preferably comprising a metal that does not easily oxidize, such as platinum, gold, or gold palladium, is formed on the bottom of the cantilever arm remote from the fixed end of the beam and positioned above and facing the gap in the signal line. A portion of the cantilever arm and an arm electrostatic plate located thereon reside above the ground line on the substrate. When a voltage is applied to the arm electrostatic plate, electrostatic forces attract the arm electrostatic plate, and thus the cantilever arm, toward the ground line on the substrate, bringing the metal contact into engagement with the separate portions of the gapped signal line, and thereby bridging the gap in the signal line.
Another example of an RF MEMS switch utilizing a cantilever actuator arm is disclosed in Loo et al., U.S. Pat. No. 6,046,659, issued Apr. 4, 2000. In Loo et al., the cantilever actuator arm comprises a multiple layer structure containing the arm electrostatic plate surrounded by insulating layers. As in Yao, the RF MEMS switch disclosed by Loo et al. provides a metal contact that bridges a gap between two portions of an RF signal line, when the switch is closed. Both Yao and Loo et al. disclose that the cantilever actuator arm is generally disposed parallel to the surface of the substrate when the RF MEMS switch is in the open position. Thus, the distance between the metal contact and the RF signal line when the RF MEMS switch is in the open position is limited to the distance between the cantilever actuator arm and the substrate along nearly the entire length of the cantilever actuator arm.
RF MEMS switches provide several advantages over conventional RF switches which use transistors. These advantages include lower insertion loss, improved electrical isolation over a broad frequency range, and lower power consumption. Since this type of switch is fabricated using existing integrated circuit (IC) processing technologies, production costs are relatively low. Thus, RF MEMS switches manufactured using micromachining techniques have advantages over conventional transistor-based RF switches because the MEMS switches function like macroscopic mechanical switches, but without the associated bulk and relatively high cost.
However, integrated RF MEMS switches are difficult to implement. Due to the proximity of the electrical contact formed on the cantilever arm to the signal line formed on the substrate, these switches tend to exhibit poor electrical isolation at high frequencies. In the RF regime, close proximity of the electrical contact and the signal line allows parasitic capacitive coupling between the contact and signal line when the switch is in the OFF-state, creating an AC leakage path for high frequency signals. These losses, which increase with signal frequency, limit the use of MEMS switches in high frequency applications.
Capacitive coupling may be reduced by increasing the separation distance between the signal line formed on the substrate and the metal contact formed on the cantilever arm. However, in the MEMS switch described above, there is a design tradeoff between the OFF-state capacitance and the switch actuation voltage. This tradeoff can be expressed mathematically. The OFF-state capacitance of the switch is given by the relation:
                              C          OFF                =                              ɛ            ⁢                                                  ⁢                          ɛ              0                        ⁢            A                    d                                    (        1        )            where A is the area of overlap between the contact and the signal line, d is the distance between the contact and the signal line, e0 is the permittivity of free space and e is the dielectric constant of the material between the contact and the signal line.
The actuation voltage of a cantilever beam in a switch as described above can be approximated by:
                              V          S          1                ≈                                            18              ⁢                                                          ⁢              E              ⁢                                                          ⁢              I              ⁢                                                          ⁢                              d                3                                                    5              ⁢                                                          ⁢              ɛ              ⁢                                                          ⁢                              ɛ                0                            ⁢                              L                4                            ⁢              w                                                          (        2        )            where E is Young's modulus of the beam material, I is the moment of inertia of the beam cross-section, and L and w are the length and width of the cantilever beam, respectively. For a cantilever beam with a uniform width w, and a thickness t, the moment of inertia is given by:
                    I        =                                            t              3                        ⁢            w                    12                                    (        3        )            and VS can be simplified to:
                              V          S                =                                            3              ⁢                                                E                  ⁡                                      (                                          d                      ⁢                                                                                          ⁢                      t                                        )                                                  3                                                    10              ⁢                                                          ⁢              ɛ              ⁢                                                          ⁢                              ɛ                0                            ⁢                              L                4                                                                        (        4        )            
Combining the above expressions (1) and (4) yieldsCOFF∝VS−2/3  (5)
Thus, in the RF MEMS switches of the type described above, increasing the separation distance between the signal line formed on the substrate and the electrical contact formed on the cantilever arm also increases the voltage required to affect electrostatic actuation of the switch, because the separation distance between the signal line and the contact is also the separation distance between the arm electrostatic plate and the ground line. The energy that must be moved through the switch control in order to activate the switch, and thus the energy dissipated by the switch, is a function of the actuation voltage. Therefore, in order to minimize the energy dissipated by the RF MEMS switch, it is desirable to minimize the actuation voltage of the switch.
Another problem with the conventional cantilever switch described above stems from the methods used to manufacture the switch. A polycrystalline silicon (or polysilicon) cantilever beam can be fabricated by first oxidizing a silicon substrate to provide a sacrificial layer, then depositing and patterning a layer of polysilicon into a long, narrow bar directly over the silicon dioxide. The beam is then separated from the sacrificial silicon dioxide layer by application of a release agent comprising a hydrofluoric acid solution, which dissolves the sacrificial layer and results in a free-standing polysilicon beam spaced apart from the substrate. The substrate is immersed in the release agent for a duration sufficient to result in release of the beam. One problem with the use of this release process for a beam in relatively close proximity to the substrate is that surface tension forces exerted by the release agent tend to pull the beam toward the substrate as the device is immersed in and pulled out of the solutions. This can cause the beam to stick to the substrate during drying, a phenomenon known as stiction.
In view of the foregoing, there is a need for a micro-electromechanical switch having improved electrical isolation and improved manufacturability, without requiring a corresponding increase in actuation voltage.